The primary objective of the Colorado Adoption Project--a longitudinal, prospective adoption study, is to assess genetic and environmental influences on behavioral development. Biological parents who relinquish their children for adoption, parents who adopt these children, matched "control" (nonadoptive) parents and the children (probands and younger siblings) in these families are studied. Adults have been administered a 3-hour battery of behavioral tests that includes measures of the major factors of specific cognitive abilities and personality. Information pertaining to family background, common medical and behavioral problems, interests and talents, and frequently used drugs is also obtained. For the children, the behavioral assessment employs standard tests of mental development, personality/temperament, motor development, and health. Environmental assessments in the adoptive and control homes are emphasized. The adopted and control probands and their younger siblings are studied yearly through 4 years of age. During the proposed 3-year continuation period, we will accomplish three major goals: (1) complete the testing of the "foundation" sample of 250 adoptive and 250 control families, including all adopted and control probands through 4 years of age; (2) test over 100 younger siblings of the adopted and control probands in the same manner as the probands; and (3) conduct analyses of the complete data set on the families and their children through 4 years of age, emphasizing multivariate, longitudinal, and biometrical-genetic approaches. Of the various methods available to study individual differences in behavioral development, the full adoption design employed in the Colorado Adoption Project is the most powerful. Continuation of this landmark study will substantially advance basic research in child development and is likely to have important implications for child-rearing practices, education, and mental health.